Platelet-activating factor (PAF) has biologic activities pertinent to asthma pathogenesis. PAF induces bronchoconstriction, increases bronchial hyperreactivity and vascular permeability, and stimulates bronchial mucus secretion and eosinophil infiltration of airways. The actions of PAF are modulated by the plasma protein PAF acetylhydrolase (PAF-AH) that hydrolyzes PAF to biologically inactive lyso-PAF. We have found that recombinant PAF-AH suppresses PAF-induced inflammation in vivo in a murine model of human asthma. This Phase 2a study will evaluate the clinical safety of rPAF-AH, administered intravenously as a single dose to subjects with stable, mild asthma. This study will also determine the effect of rPAF-AH on allergen-induced early and late phase asthmatic responses.